The present invention relates to optics and, more particularly, to a method and apparatus for designing optical surfaces capable of providing a light beam having a predetermined profile.
Optical beam-shaping systems generally are arrangements of optical elements by which a bundle of rays of light irradiated upon such elements is modified in a defined way with respect to its beam parameters. It is required in the main cases of practical application that a bundle of rays has a defined geometric shape with respect to its cross section, e.g., a circular, rectangular or lattice-like shape or the like, and an intensity profile defined across its cross section. Both properties frequently have to be influenced simultaneously. For example if the light source delivering the incoming bundle of rays supplies a bundle of rays with irregular intensity distribution and irregular geometric dimensions, but defined specifications have to be satisfied for the outgoing bundle of rays of the beam-shaping system with respect to its properties.
The problem of intensity control for light beams is important in many fields. In laser applications, for example, particular importance is attached to both the quality of the beam produced by a laser and the shaping of that beam for the desired use. A laser device generally produces a beam of coherent light that has a wavefront of relatively small cross-section. In spite of the small cross-section and the coherency of the beam, the wavefront of a laser typically has a non-uniform power distribution that is stronger in the center than at the outer edges. The laser output beam quality and shape determine the quality, quantity and efficiency of work piece machining. The power variation may be between five and ten percent and if not reshaped to produce a uniform distribution may result in uneven machining over the work piece surface. Furthermore, to make use of the beam, it is often necessary to expand the cross-sectional area of the beam, thereby spreading the non-uniformity across a larger wavefront. This is because when conventional lenses are used to expand the beam, the non-uniform power distribution of the wavefront is carried through to the expanded beam.
Intensity control for light beams is also required in the area of communication whereby optical signals have to be transmitted between various optical components. A conventional light-emitting module incorporated in an optical communications system generally includes a light source (e.g., a laser diode), an optical fiber and a lens interposed between the light source and optical fiber for converging the light beam onto the core of the optical fiber. It is recognized that the communication efficiency depends on the ability of the lens to provide the optical signal passing with the proper intensity profile so as to reduce coupling losses.
An additional application in which it is required to control the intensity profile of a light beam is optical scanning. Optical scanners, such as bar code scanners, typically make use of light from laser diodes which are moved to provide the scanning beam. Such diodes are robust and relatively inexpensive, but they suffer from the disadvantage that the beam emerging from a laser diode is astigmatic. When a bar code symbol is to be scanned it is generally desirable for the beam width to be relatively small at the point at which it impinges upon the bar code symbol, to provide proper discrimination between the bars and spaces. On the other hand, it is desirable to have the perpendicular dimension relatively large to minimize noise. It is therefore desired to control the intensity profile of such optical scanners to allow noise free reading with minimal astigmatism.
In lighting modules, such as those used in the theater, television, touring productions and architectural applications, it is oftentimes desired to control the intensity profile of a light beam, as well as the hue, saturation, and color profile to obtain a particular artistic effect.
The activity in the area of light beam intensity profile has grown considerably over the past two decades. Known designs for shaping the light beam are typically based on symmetry assumptions. Thus, light beams have traditionally been assumed to have an intensity profile that depends on a single coordinate, a radial intensity profile, a product of one-dimensional profiles, and the like. Such assumptions have heretofore reduces the design problem to one spatial dimension which enabled the formulation of a solvable mathematical equation.
One method of designing a beam-shaping system without imposing symmetry assumptions is disclosed in an article by T. Glimm and V. Oliker entitled “Optical design of two-reflector systems, the Monge-Kantorovich mass transfer problem and Fermat's principle”, published in Indiana Univ. Math. J. 53:1255-1277, 2004. Glimm et al. design a beam-shaping system for transforming an incoming beam having a plane wave front with given intensity into an outgoing beam having a plane front with prescribed output intensity. The beam-shaping system includes a first reflector and a second reflector. An incoming beam is reflected off the first reflector which transforms it into an intermediate beam, propagation in the direction of the second reflector. The intermediate beam impinges on the second reflector, which transforms it into an outgoing beam of the desired shape. Unlike other methods, Glimm et al. do not impose a priori assumptions on the symmetry of the incoming light beam and the two-reflector beam-shaping system does not have to be symmetric. However, since the two reflectors are non-planar, they have to be accurately aligned to provide the desired output intensify profile. Any misalignment of the reflectors varies the intensity profile of the outgoing light beam and results in a reduced efficiency. The alignment requirement introduces complications in the fabrication process of the system.
There is thus a widely recognized need for, and it would be highly advantageous to have a method and apparatus for designing transmissive optical surfaces, devoid of the above limitations.